Infection with the Apicomplexan parasite Plasmodium falciparum causes the most severe form of malaria in humans. Efficient invasion of human erythrocytes by P. falciparum is associated with virulence of the disease. The overall objective of this proposal is to elucidate the molecular mechanisms by which P. falciparum invasive forms (known as merozoites) target and invade host erythrocytes. P. falciparum invades human erythrocytes using multiple and specific ligand-receptor interactions defined as invasion pathways. A family of proteins has been identified in P. falciparum, that are homologous to P. vivax reticulocyte proteins that bind to reticulocytes with high affinity, and have been termed the P. falciparum Reticulocyte Binding Protein homologue (PfRh) proteins. The PfRh proteins are localized at the apical end of the merozoite and specific antibodies raised against these proteins are inhibitory to invasion. We have used targeted gene disruptions to demonstrate that one member of this family, PfRh2b, plays a central role in directing P. falciparum merozoites to use specific receptors on the erythrocyte surface. We hypothesize that the PfRh proteins are involved in the sensing the apical orientation of the P. falciparum merozoite on the erythrocyte surface through the recognition of specific receptors. This recognition can be mediated either through direct binding of the Pf'Rh ectodomain to the receptor, or indirectly though the formation of a complex with other parasite adhesions. Following this initial engagement, a signal is transmitted intracellularly to downstream effector molecules involved in the ordered process of invasion through the function of the cytoplasmic tail domain. We will use a combination of molecular, genetic and biochemical means to determine the precise role of specific domains of the PfRh2b protein, which includes a region of the ectodomain, the transmembrane region and the cytoplasmic tail domain, in the invasion process. Our experimental design exploits our ability to genetically manipulate P. falciparum. To test our hypotheses our specific aims are: i) a reverse genetics analysis of the roles of the specific domains of the PfRh2b protein in the invasion process and in organellar targeting, and ii) the identification of P. falciparum proteins interacting with these domains. These studies will directly lead to a better understanding of the molecular mechanisms underlying the invasion of erythrocytes by P. falciparum, to facilitate our long-term goal of using such knowledge towards the design of effective vaccine and drug strategies to combat the disease.